<!DOCTYPE html>
<html>

<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <script src="vue.js"></script>
</head>

<body>
  <div id="app">
    <!-- input中的v-model用于在表单控件元素上创建双向数据绑定 -->
    <input type="text" v-model="cityName">
  </div>
  <script>
    var vm = new Vue({
      el: '#app',
      data: {
        cityName: 'shanghai'
      },
      // 使用watch监听cityName变化
      watch: {
        cityName(newName, oldName) {
          console.log(newName, oldName)
        }
      }
    })
  </script>
</body>

</html>